Epitaxial VO2 Films Research Articles

Low-temperature growth of high-quality VO2 epitaxial film on c-plane sapphire by reactive magnetron sputtering

Low-temperature growth of high-quality VO2 epitaxial film on c-plane sapphire by reactive magnetron sputtering

Separation of terahertz and DC conductivity transitions in epitaxial vanadium dioxide films

Terahertz (THz) light, probing electronic responses near the Fermi level, is closely linked to DC transport. In this study, we investigate the in situ THz and DC conductivity of epitaxial VO2 films near the heating-induced insulator–metal transition. We find that the THz conductivity transition occurs at lower temperatures than the DC conductivity transition, with the separation increasing with film thickness and crystallization quality. Analysis of the complex conductivity spectra attributes this separation to released carrier confinement at THz frequencies. Our findings contribute to a deeper understanding of electron dynamics in VO2 films.

Fabrication of epitaxial V2O3 thin films on Al2O3 substrates via mist chemical vapor deposition

Epitaxial V2O3 films, which undergo a metal–insulator transition at 155 K, exhibit a drastic change in resistivity. Consequently, they are expected to be used in current-driven switching devices and smart switching devices for the protection of superconducting coils. However, conventional fabrication methods for epitaxial V2O3 films require a high vacuum, resulting in high processing costs and low productivity. In this study, we report the fabrication of epitaxial V2O3 films on R-, C-, and A-plane sapphire substrates via mist chemical vapor deposition (mist CVD), which does not require a high vacuum. Consequently, it facilitates the fabrication of thin films at a low cost with high productivity. Deposition at various temperatures reveal that epitaxial V2O3 films can be grown on sapphire substrates, even under atmospheric pressure, and the optimal temperature for epitaxial growth is 823 K. All the films grown at 823 K exhibit a clear metal–insulator transition, demonstrating a drastic change in the resistivity at approximately 155 K. The film on C-plane sapphire exhibits a lower transition temperature (129 K) than those on R- and A-plane sapphire substrates. The fabrication of high quality films via the low cost mist CVD process can enable the mass production of V2O3-based devices.

Correlation of metal-to-insulator transition and strain state of VO2 thin films on TiO2 (110) substrates

We explore the possibility of tuning the metal-to-insulator transition (MIT) of crystalline VO2 thin films by strain engineering. We deposit high-quality VO2 epitaxial films of different thicknesses on TiO2 (110) substrates by pulsed laser deposition. The strain state of the deposited film varies with its thickness. This allows us to correlate the MIT characteristics with the strain state of the VO2 film by a careful characterization of the structural and electrical properties. Thin VO2 films on TiO2 (110) substrates are almost fully strained up to thicknesses of about 20 nm and exhibit tensile strain along the c axis of the (high-temperature) metallic rutile phase leading to an increase in the MIT temperature by as much as 30 °C in comparison to the almost fully relaxed 300 nm-thick VO2 film. The strain gradient within the thicker samples leads to a continuous serial switching of layered regions of the VO2 film from the insulating to the metallic state with increasing temperature.

Microstructural and electrical investigation of polymorph stabilization and multistate transition in interface engineered epitaxial VO2 films

Modulation of metal to insulator transition (MIT) in vanadium dioxide (VO2) remains one of the most important cornerstones for its wide scale applications in electrical devices. Incorporating VO2 within and beyond room temperature ranges requires energy efficient systems with high device performance and low transition losses. To achieve this performance, we studied pulsed laser deposition (PLD) grown VO2 films on sapphire substrate, with and without titanium dioxide (TiO2) buffer layer. An insight into microstructural modification and MIT was provided using in-situ characterization techniques. We observed that misfit strain and external stimuli induces polymorphs and phases like VO2 (M2), VO2 (T) and V2O3 in the epitaxial films along with higher stabilization of intermediate M2 for the TiO2 system. To understand the effect of interface engineering on transition temperature and voltage, in-plane and out-of-plane device architectures were fabricated and characterized using electrical measurements. We discovered that TiO2 interface enables realization of two step electrical transition, Tc close to room temperature and lowering of transition voltage and work function. Also, using DFT calculation, we studied the effect of substrate misfit strain on structural and electronic properties of polymorphs. Since M1 and R have different structures, the strain experienced before and after transition on the same substrate will be different. Using the orientation relation of PLD grown films, we predict the MIT temperature, validating the Tc observed through experimental electrical measurements.

Multiphysical Field Modulated VO2 Device for Information Encryption.

In the information explosion society, information security is highly demanded in the practical application, which raised a surge of interest in designing secure and reliable information transmission channels based on the inherent properties of emerging devices. Here, an innovative strategy to achieve the data encryption and reading during the data confidential transmission based on VO2 device is proposed. Owing to the specific insulator-to-metal transition property of VO2 , the phase transitions between the insulator and metallic states are modulated by the combination of electric field, temperature, and light radiation. These external stimulus-induced phase diagram is directly associated with the defined VO2 device, which are applicable for control the "0" or "1" electrical logic state for the information encryption. A prototype device is fabricated on an epitaxial VO2 film, which displayed a unique data encryption function with excellent stability. The current study not only demonstrated a multiphysical field-modulated VO2 device for information encryption, but also supplied some clues for functional devices applications in other correlated oxide materials.

Impact of the uniaxial strain on terahertz modulation characteristics in flexible epitaxial VO2 film across the phase transition.

Exploring flexible electronics is on the verge of innovative breakthroughs in terahertz (THz) communication technology. Vanadium dioxide (VO2) with insulator-metal transition (IMT) has excellent application potential in various THz smart devices, but the associated THz modulation properties in the flexible state have rarely been reported. Herein, we deposited an epitaxial VO2 film on a flexible mica substrate via pulsed-laser deposition and investigated its THz modulation properties under different uniaxial strains across the phase transition. It was observed that the THz modulation depth increases under compressive strain and decreases under tensile strain. Moreover, the phase-transition threshold depends on the uniaxial strain. Particularly, the rate of the phase transition temperature depends on the uniaxial strain and reaches approximately 6 °C/% in the temperature-induced phase transition. The optical trigger threshold in laser-induced phase transition decreased by 38.9% under compressive strain but increased by 36.7% under tensile strain, compared to the initial state without uniaxial strain. These findings demonstrate the uniaxial strain-induced low-power triggered THz modulation and provide new insights for applying phase transition oxide films in THz flexible electronics.

Embedding epitaxial VO2 film with quality metal-insulator transition to SAW devices

Epitaxial VO2 films grown by pulsed laser deposition (PLD) method with superior phase transition related switching characteristics are successfully embedded to SAW devices using concept of the “impedance loaded SAW sensor”. A resistance of VO2 sensor abruptly drops from 0.7 M[Formula: see text] to 90 [Formula: see text] when it is heated above [Formula: see text]65[Formula: see text]C and shows a narrow hysteresis loops when switching. Two designs of SAW devices are examined in which RF signal is reflected back from interdigital transducer (IDT) or a surface acoustic waves (SAW) is transferred through a coupler and the RF response is altered 2 and 3 times correspondingly upon the phase transition in VO2. In the proposed devices with external load, a SAW does not propagate via VO2 film and therefore is not attenuated which is beneficiary for wireless applications. Additionally, a SAW phase shift as great as 50[Formula: see text] is induced to the SAW transferred through the coupler due to the phase transition in VO2. The proposed approach may boost the development of remotely controlled autonomous sensors, including those based on VO2 metamaterials and hybrid plasmonic structures for near IR/middle IR and sub-THz/THz applications.

Reversible control of the metal-insulator transition in V2O3 thin films through plasma hydrogenation

We investigate the effect of hydrogen plasma treatment on the metal-insulator transition (MIT) of epitaxial V2O3 films grown on c-Al2O3. The films were exposed to plasma at constant power for varying intervals. With increasing hydrogenation the films display a suppression of the MIT magnitude and temperature due to neutralization of structural defects and passivation of unpaired bonds by incorporation of atomic hydrogen as supported by relaxation in strain by XRD and Raman spectroscopy analysis, while stabilizing the metallic phase due to reduction in Peierls dimerization of V-V bonds. Heating to 350-400 °C the electrical characteristics of the film in the as-grown state are regained showing the possibility of reversibly controlling the MIT characteristics.

Nanoscale mapping of temperature-dependent conduction in an epitaxial VO2 film grown on an Al2O3 substrate.

Vanadium dioxide (VO2) is one of the extensively studied strongly correlated oxides due to its intriguing insulator–metal transition near room temperature. In this work, we investigated temperature-dependent nanoscale conduction in an epitaxial VO2 film grown on an Al2O3 substrate using conductive-atomic force microscopy (C-AFM). We observed that only the regions near the grain boundaries are conductive, producing intriguing donut patterns in C-AFM images. Such donut patterns were observed in the entire measured temperature range (300–355 K). The current values near the grain boundaries increased by approximately two orders of magnitude with an increase in the temperature, which is consistent with the macroscopic transport data. The spatially-varied conduction behavior is ascribed to the coexistence of different monoclinic phases, i.e., M1 and M2 phases, based on the results of temperature-dependent Raman spectroscopy. Furthermore, we investigated the conduction mechanism in the relatively conductive M1 phase regions at room temperature using current–voltage (I–V) spectroscopy and deep data analysis. Bayesian linear unmixing and k-means clustering showed three distinct types of conduction behavior, which classical C-AFM cannot resolve. We found that the conduction in the M1 phase regions can be explained by the Poole–Frenkel mechanism. This work provides deep insight into IMT behavior in the epitaxial VO2 thin film at the nanoscale, especially the coexistence and evolution of the M1 and M2 phases. This work also highlights that I–V spectroscopy combined with deep data analysis is very powerful in investigating local transport in complex oxides and various material systems.

Strong strain gradients and phase coexistence at the metal-insulator transition in VO2 epitaxial films

The proximity of a thermodynamic triple point and the formation of transient metastable phases may result in complex phase and microstructural trajectories across the metal-insulator transition in strained VO2 films. A detailed analysis using in-situ synchrotron X-ray diffraction unveils subtle fingerprints of this complexity in the structure of epitaxial films. During phase transition the low-temperature monoclinic M1 phase is constrained along the {111}R planes by the coexisting high-temperature R phase domains, which remain epitaxially clamped to the substrate. This geometrical constraint induces counteracting local stresses that result in a combined tilt and uniaxial in-plane compression of M1 domains, and a concomitant anomalous cR-axis elongation. This mechanism progressively transforms the M1 phase into the transitional triclinic phase (T), and ultimately into the monoclinic M2 phase, generating strong strain and tilt gradients that remain frozen after the complete transformation of the R phase upon cooling to RT. The transformation path of VO2 films, the complex competition between stable and metastable VO2 polymorphs and its impact on the structure of the low temperature monoclinic state, provide essential insights for understanding the electronic and mechanical properties of the films at the nanoscale, as well as to control their use in functional devices.

Manipulating the anisotropic phase separation in strained VO2 epitaxial films by nanoscale ion-implantation

Manipulating the strain induced poly-domains and phase transition in a correlated oxide material is important for high performance device fabrication. Though the electronic transport in the strained oxide film at macroscopic scales can be directly measured, the anisotropic electronic state and the controllable phase separation cross the insulator-to-metal transition within nanoscale size are still elusive. Here, we selected a VO2 crystal film as prototypical oxide and achieved the manipulation of anisotropy electronic phase separation via injecting a He+ nanobeam into the VO2 film at room temperature. In addition, this nanoscale phase separation was directly visualized by infrared near-field imaging measurements, showing the pronounced and unique cR-axis dependent anisotropy on the VO2 surface. Our results offered insight toward understanding the anisotropic nanoscale phase separation in strained metal oxide films.

Strained interface layer contributions to the structural and electronic properties of epitaxial V2O3 films

We report on the transport properties of epitaxial vanadium sesquioxide (V2O3) thin films with thicknesses in the range of 1 to 120 nm. Films with the thickness down to nanometer values reveal clear resistivity curves with temperature illustrating that even at these thicknesses, the films are above the percolation threshold and continuous over large distances. The results reveal that with the reducing thickness, the resistivity of the films increases sharply for thicknesses below 4 nm and the metal-insulator transition (MIT) is quenched. We attribute this increase to a strained interface layer of thickness ∼4 nm with in-plane lattice parameters corresponding to the Al2O3 substrate. The interface layer displays a suppressed MIT shifted to higher temperatures and has a room temperature resistivity 6 orders of magnitude higher than the thicker V2O3 films.

Delicate tuning of epitaxial VO2 films for ultra-sharp electrical and intense IR optical switching properties

Epitaxial VO2 films on r-sapphire substrates with ideal composition, dense structure and superior functional properties were obtained by water-assisted MOCVD from vanadyl hexafluoroacetylacetonate followed by the annealing under optimized conditions. The obtained films exhibit both intense optical switch properties in IR region and unique MIT characteristics: change in resistance by 4.6 orders in the temperature range 40–80 °C, hysteresis width less 0.7 °C and transition width 1.5 °C. The proposed synthetic approach opens new opportunities to fabricate highly sensitive and ultrafast switches for optical, electronic and biomedical devices.

Deposition-Temperature-Mediated Selective Phase Transition Mechanism of VO2 Films

A clear experimental explanation of the contribution of Mott and Peierls transitions to the insulator–metal transition (IMT) characteristics in vanadium dioxide (VO2) is still lacking. Examining the crystal and electronic structures of epitaxial VO2 films grown at various deposition temperatures, a Mott or a Peierls transition was observed. The VO2 film deposited at 500 °C showed suppressed Peierls transition characteristics because of the large in-plane compressive strain in the insulating phase. The VO2 films deposited at 600 and 650 °C had a higher IMT temperature because of the relaxation of both the in-plane and out-of-plane strain, and there were abundant V4+ states. Therefore, it was related to a collaborative Mott–Peierls transition. Finally, the VO2 film deposited at 720 °C showed a suppressed Mott transition because of the abundance of V3+ states in the insulating phase. Furthermore, an analysis of the electronic structure of the insulating and metallic phases using in situ X-ray photoelectron spectroscopy and X-ray absorption spectroscopy provide a complete band diagram to support the above explanation of the deposition-temperature-dependent IMT characteristics.

(Invited) A Reconfigurable Remotely Epitaxial VO2 Electrical Heterostructure

VO2 has been regarded as an archetypal phase transition building block with superior metal-insulator transition characteristics. However, reconfigurable VO2-based heterostructure and the associated devices are rare due to the fundamental challenge in integrating high quality VO2 in technologically important substrates. In this report, for the first time, we show the remote epitaxy of VO2 and the demonstration of a vertical diode device in graphene/epitaxial VO2/single crystalline BN/graphite structure with VO2 as a reconfigurable phase change material and hexagonal boron nitride (h-BN) as an insulating layer. By diffraction and electrical transport studies, we show that the remote epitaxial VO2 films exhibits higher structural and electrical quality than direct epitaxial one. By high resolution transmission electron microscopy and Cs-corrected scanning transmission electron microscopy we show that graphene buffered substrate leads to less strained VO2 film than bare substrate. In the reconfigurable diode, we find that the Fermi level change and spectral weight shift along with metal-insulator transition of VO2 could modify the transport characteristics. The work suggests the feasibility of developing single crystalline VO2-based reconfigurable heterostructure with arbitrary substrates and sheds light on designing novel adaptive photonics and electrical devices and circuits.

Controllable phase transition temperature by regulating interfacial strain of epitaxial VO2 films

Modulation of the vanadium oxide epitaxial film metal insulator transition process through interfacial strain is a novel approach for controlling the properties of epitaxial vanadium oxide films, improving its scope for device applications. In this work, vanadium oxide films of various thickness were prepared and the effects of strains on the transition temperature were investigated. The vanadium oxide films exhibited compressive or tensile interfacial strain along their c-axis, the transition temperature varied in a range of 310–370 K owing to the effects of the interfacial strains. The tensile strain along the c-axis is correlated with an increase in transition temperature; however, the compressive strain along the c-axis leads to a decrease in transition temperature. The results show that the metal insulator transition process of vanadium oxide thin films can be regulated by interfacial strain. This work shows the feasibility of engineering the phase transition of epitaxial vanadium oxide films or the related devices through strain manipulating, which may be of great importance in practical applications.

Strain engineering on the metal-insulator transition of VO2/TiO2 epitaxial films dependent on the strain state of vanadium dimers

In this letter, a series of vanadium dioxide (VO2) epitaxial films were deliberately deposited on TiO2 substrates with different orientations [(001), (110), and (101)], in an attempt to gain insights into the strained VO2 epitaxial film. We found in-plane [100] and [1-10] directions, obviously anisotropic metal-insulator transition (MIT) in (110)-oriented VO2 films. In combination with synchrotron radiation high-resolution x-ray diffraction characterizations, electronic transport data reveal that the critical temperature of MIT depends on the strain state of the dimeric vanadium atomic chain along the c axis of the rutile phase. The anisotropy of MIT is closely related to the orientation of the VO2 films, which is caused by the varied orientation configuration of V-V atomic chain dimerization in the films. Soft x-ray absorption spectroscopy results further indicate that this anisotropy may be driven by the directional hybridization of O 2p and V 3d orbitals with respect to the orientation of VO2 thin films. The polarization-dependent V L-edge and O K-edge XAS data suggest that the elongation of the apical V-O bond length increases the p-d orbital overlap; thus, the energy level of the d// orbital is raised relative to that of the π* orbital. These anisotropic MIT behaviors will help us to understand how the strain engineering depends on the strain state of vanadium dimers in VO2 films.

A Reconfigurable Remotely Epitaxial VO2 Electrical Heterostructure.

The reconfigurability of the electrical heterostructure featured with external variables, such as temperature, voltage, and strain, enabled electronic/optical phase transition in functional layers has great potential for future photonics, computing, and adaptive circuits. VO2 has been regarded as an archetypal phase transition building block with superior metal-insulator transition characteristics. However, the reconfigurable VO2-based heterostructure and the associated devices are rare due to the fundamental challenge in integrating high-quality VO2 in technologically important substrates. In this report, for the first time, we show the remote epitaxy of VO2 and the demonstration of a vertical diode device in a graphene/epitaxial VO2/single-crystalline BN/graphite structure with VO2 as a reconfigurable phase-change material and hexagonal boron nitride (h-BN) as an insulating layer. By diffraction and electrical transport studies, we show that the remote epitaxial VO2 films exhibit higher structural and electrical quality than direct epitaxial ones. By high-resolution transmission electron microscopy and Cs-corrected scanning transmission electron microscopy, we show that a graphene buffered substrate leads to a less strained VO2 film than the bare substrate. In the reconfigurable diode, we find that the Fermi level change and spectral weight shift along with the metal-insulator transition of VO2 could modify the transport characteristics. The work suggests the feasibility of developing a single-crystalline VO2-based reconfigurable heterostructure with arbitrary substrates and sheds light on designing novel adaptive photonics and electrical devices and circuits.

Tuning critical phase transition in VO2 via interfacial control of normal and shear strain

Interface strain plays a key role in creating the emergent functional properties of heteroepitaxially correlated materials. Strain that originates from the lattice mismatch of thin films and substrates has been widely studied to support the creation of desired functionalities. However, the shear strain induced by the symmetry mismatch of heterostructures has rarely been considered. Here, we report evidence of twin domains of stabilized vanadium dioxide (VO2) epitaxial films grown on sapphire substrates with a miscut along the a-plane. A systematic investigation of lattice variations, including lattice rotations and lattice distortions, reveals that both normal strain and shear strain can be manipulated by vicinal sapphire surfaces using different miscut angles. Consequently, the critical phenomenon of metal-insulator transitions (MITs) in VO2 epitaxial films is strongly coupled with lattice variations. A significantly sharpened MIT transition, over four orders of magnitude in resistance change, is also achieved by controlling interfacial shear strain. Our results demonstrate that the degree of freedom of shear lattice deformation opens the door to fine-tune the critical properties of heterostructures of strongly correlated oxides to aid in the development of electronic devices.